1. Field of the Invention
The present invention relates generally to the field of robotic tool changers and more particularly to high power electrical contacts for robotic tool changers.
2. Description of Related Art
Robots have been used for many years in industrial assembly line applications to perform repetitive tasks very precisely without the need for human operation, interaction, or supervision. For instance, robots are commonly used in the automotive industry to perform a number of tasks such as material handling and spot-welding of automobile bodies.
In many cases, a robot has a dedicated function which is the only function it performs. In certain circumstances, however, it is desirable for a single robot to perform various different tasks. For instance, it might simplify the production line to provide a single robot capable of spot-welding, palletizing heavy loads, and engaging in metal removal operations. In order to provide this flexibility, a robotic tool changer is used to give the robot access to the tools it needs to perform its various tasks.
The robotic tool changer is a device that provides the means for a robot or other automatic machine to easily exchange tools as required for different jobs. As shown in FIG. 1, a robotic tool changer is usually made up of two halves--a master assembly 1 and a tool assembly 2. A single master 1 assembly is permanently attached to the arm 3 of the robot, while several tool assemblies 2 are associated with the various tools (not shown) to be used by the robot. When the robot needs a certain tool, it simply couples its master assembly 1 to the tool assembly 2 of the appropriate tool. When the master and tool assemblies 1 and 2, respectively, are coupled together, they provide the mechanical, electrical, and pneumatic connections and signal interfaces which permit the robot to perform the particular task.
Some tools that a robot may use to perform a particular task, such as spot-welding, require the robot to supply high power (10-500 Amps at 100-1000 Volts) to the tool. For these applications, the master and tool assemblies of the robotic tool changers are each provided with a high power module 4. Such high power modules 4 provide the means for transferring the necessary high electrical energy between master and tool assemblies 1 and 2. The high power modules 4 may be integral with the master and tool assemblies 1 and 2, or they may be removably attached to the master and tool assemblies 1 and 2.
The electrical connection between the master and tool assemblies 1 and 2 is made by high power electrical contacts 5 integrated into the high power modules 4. Typically, each high power module 4 contains multiple electrical contacts 5 extending into openings within the module's electrically insulating housing. In order to maximize the surface area available for making electrical contact, the contacts 5 usually have a large, flat contact surface 7, as shown in FIG. 2. During operation, the high power modules 4 (FIG. 1) of the master and tool assemblies 1 and 2 are mated together so that their sets of contacts 5 align and abut. Since the electrical contacts 5 are electrically connected to heavy duty electrical cables, the transfer of high power may be realized.
Under ideal conditions, the useful life of prior art electrical contacts 5 is on the order of several million cycles, after which the contacts 5 must be replaced. However, during normal usage on an assembly line, the high power electrical contacts 5 are subjected to a number of environmental contaminants which may drastically shorten their life. When the master and tool assemblies 1 and 2 of the robotic tool changer are not connected, the contacts 5 of the high power modules 4 are left exposed. Particularly, the contacts 5 on the tool assembly's high power module 4 are left exposed and upright with their flat contact surfaces 7 essentially horizontal during non-use. During the period of exposure, the high power electrical contacts 5 may encounter contaminants such as dirt, dust, grease, water, and other foreign objects. Because the contacts 5 have flat contact surfaces 7, the contaminants will tend to come to rest on the flat contact surface 7 rather than sliding off onto another portion of the high power module 4.
If other welding operations are being conducted nearby, the high power electrical contacts 5 are also subjected to welding slag. Welding slag is composed of minute particles of molten metal, and oxides thereof, that are generated and sprayed about in the spot-welding process, usually as bright sparks. In the molten state, welding slag may bond to the electrical contacts 5 of the high power modules 4. In this situation, the welding slag may then solidify or become welded onto the high power electrical contacts 5. The molten welding slag may also solidify in mid-air, thereafter landing on the high power electrical contacts 5 in a hardened state.
Later, when the master and tool assemblies 1 and 2 are mated so that an electrical connection is made between the high power modules 4, the welding slag or other contamination on the contacts 5 may cause the electrical contacts 5 to overheat, melt, arc, pit, and/or corrode. Specifically, when an electrical contact 5 contaminated with welding slag is mated to another contact 5, a large portion of the current passing through the contact 5 connection tends to divert through the welding slag. With such a concentration of high power across the welding slag, the welding slag may heat to the point of fusion, ablation, or oxidation. This intense heat may cause the contact surface 7 (FIG. 2) of the electrical contacts 5 to become pitted or even welded together. Pitting results in an uneven contact surface that reduces the surface area of the contacts 5 actually touching each other. This increases the resistance of the circuit, thereby generating additional unnecessary heat and reducing the power available to the robot.
Therefore, once an electrical contact 5 becomes contaminated and damaged, the contact 5 must be replaced. Previously, replacement of a contaminated electrical contact 5 involved disassembly of the entire high power module 4, disconnection of the electrical cables, removal and replacement of the contact 5, reconnection of the cables, and reassembly of the module 4. Replacement, therefore, was a time-consuming, labor-intensive, and expensive process.
The inventors of U.S. Pat. No. 5,460,536, which issued on Oct. 24, 1995 to Applied Robotics, Inc., recognized the problems associated with replacement of high power electrical contacts on robotic tool changers. Their patented solution to the problem was to provide electrical contacts which can be replaced without having to disassemble the high power modules which house the contacts.
While providing easily replaceable electrical contacts is one way of lessening the impact of contaminated and damaged contacts, replacing such contacts still requires taking the robot out of service for a period of time. Obviously, this leads to production line down-time and reductions in overall efficiency. It would be more advantageous to provide a high power electrical contact which, in addition to being easily replaceable, is less susceptible to contamination and damage in the first place, therefore needing replacement less often than traditional contacts.